Optical memories of the type having large amounts of digital data stored by the optically modifying characteristics of a film or thin layer of photosensitive material and accessed by electrical addressing without mechanical movement, volumetric storage memories, have been proposed but have not resulted in wide spread commercial application. The interest in such optical recording and retrieval technology is due to its capability for fast retrieval of large amounts of data compared to that of existing electro-optical mechanisms such as optical discs and magnetic storage such as tapes and magnetic discs, all of which require mechanical motion of the storage medium.
For example, in the case of optical disc memories, it is necessary to spin the record and move a read head in a radial motion across the disc to retrieve the data, which is output in serial fashion. The serial accessing of data generally requires transfer to a buffer or to the solid state random access memory of a data processor in order to accommodate high-speed data addressing and other data operations of modern computers. Solid state Read-Only-Memory (ROM) and Random-Access-Memory (RAM) can provide the relatively high access speeds that are sought, but the cost, size, and heat dissipation of such devices when expanded to relatively large data capacities limit their applications.
Examples of efforts to provide the relatively large capacity storage and fast access of an optical memory of the type that is the subject of this invention are disclosed in the patent literature. Most share a similarity of optical system design with the present invention, particularly in the source array, lenslet array, and detector array but are limited to the storage of one "page" of data at each storage location within the volumetric storage media. Much of the early work was done by James T. Russell, who disclosed a method of writing and reading serially from a two dimensional photographic record by mechanically moving a set of microlenses and a laser beam to extract the digital data in U.S. Pat. No. 3,806,643, Photographic Records of Digital Information and Playback Systems Including Optical Scanners, (1974), and U.S. Pat. No. 3,885,094, Optical Scanner, (1975).
In U.S. Pat. No. 3,898,005, High Density Optical Memory Means Employing A Multiple Lens Array, a page of information is first displayed on a "page composer" that is the source of light for writing the digital data. A single mechanical shutter is then opened allowing light to pass through one of a plurality of lenslets in an array and thereby reimage the data from the page composer onto the film. The aperture is then closed and mechanically moved to another lenslet in the array. A different page of digital information is then displayed on the page composer, the shutter is opened, and another page of data is recorded on the film.
The development of two-dimensional optical storage continued with devices such as that disclosed in U.S. Pat. No. 3,996,570, Optical Mass Memory, in which a means of recording and writing multiple storage fields "pages" within a two dimensional storage area of a recording medium is detailed. A non-mechanical means of deflecting a laser beam is described which writes digital information into a storage field "page." In the read operation, one element of an LED array is used to illuminate a particular storage field "page" and the digital information is reimaged onto a detector array.
Volumetric storage development has been limited because many of the optical memory devices have lenses or other optical structures not capable of providing the requisite data density. The optical resolution of the data image by these lens systems does not result in sufficient data density and data rate to compete with other forms of digital memory. Although certain lens systems, such as microscope objectives, used in other fields are theoretically capable of the needed resolutions, such lens combinations are totally unsuited for reading data stored in closely spaced data fields. Another difficulty encountered with existing designs is the practical effect of temperature and other physical disturbances on the mechanical relationship between the data film or layer, the lens assemblies, and the optical sensors that convert the optical data to electrical signals. For example, the thermal expansion effects of even moderate density optical memories of this type can cause severe misregistration between the optical data image and the read-out sensors. Similar difficulties are encountered in the required registration between the recording process and the subsequent reading operations. Intervening misregistration of the high-density optical components can cause significant data errors if not total loss of data.